Sanding and Finishing MDF

How much sanding is involved in getting a nice finish on MDF? September 29, 2009

I am wondering what other folks are charging for painted finishes. We use an outside finisher who charges $12.00/ft for a painted finish. This is a satin, opaque pre-cat lacquer finish on MDF. That adds up quickly, but in addition, our finish materials supplier tells us that the only way to get a really good finish on MDF is to sand it to 400g. We spend a huge amount of time sanding; 150,220,280,320,400g. No wide belt, only 6' Festool R/O sanders. Any suggestions on how to lower the cost and still get a top notch finish?

Forum Responses
(Finishing Forum)
From contributor A:
What are you sanding? Routed panels or other machined areas of the MDF or the factory flat surfaces? What does your finisher say about the sanding?

From the original questioner:
Everything is flat slabs. We are switching over to a new finisher and I have not yet been able to pin him down on exactly how he would want the parts prepared. It could be that he does not require sanding to 400g. The bigger question is what do I need to do to get the best finish, and how much can I expect that to cost. My old finisher charged $7.50/ft, but the quality was not there.

From contributor B:
The flat unmachined panels should not be sanded at all. The machined or abraded areas should be sanded to 240, spot primed, then sanded to 320 and then primed. Sanding any substrate past 240 is a fool's errand. The primer may have adhesion problems. Primer is a hell of a lot easier to sand than wood or MDF.

Glass finish on a sheet of MDF:

do not sand(this is your prep work)
spray 1st coat primer
sand 320
spray 2nd coat primer
sand 400
spray 1st top coat
sand 400
spray 2nd top coat

Find a new finisher.

From contributor C:
You shouldn't have to sand beyond 220 grit. The nice sanding is on the primer. You need a good high build primer and he needs to let it dry thoroughly. That will eliminate a lot of the sanding scratches that can telegraph through.

From contributor D:
We burnish rather than sand routed areas with 400g. There is little to no sanding on-non machined areas. Prime with MLC Level and coat with MLC Stealth. This is a great finish for MDF. When you say $12.00/foot, are you talking running foot of cabinet, square foot of finish or what?

From the original questioner:
$12.00 sq/ft (1 side, double for 2 sided)

From contributor E:
Finishing costs vary just like the quality of a finish can vary. Simply because someone may charge a pretty penny does not necessarily mean that you will get a perfectly shiny penny in return. More important is the consistency of the finish that you get from the finisher, his/her ability to meet the schedule that you have set, his/her skill at finishing the particular project, etc...

I would not just judge a finisher by his/her cost for the job but by a few of the other aspects of finishing that I mentioned. I personally think that sanding an MDF panel or for that matter anything to 400 grit is overkill and as others have mentioned it could cause an adhesion problem, either right away ,or down the road when the finish has completely cured.

I can't say one way or another if you are paying too much money to have your cabinets finished but as a seasoned finisher myself I can say that quality coatings cost, quality finishes cost, and the satisfaction that you can rely on your finisher to come to bat for you each and every time you need him/her to also costs. Good finishers are hard to come by and once you find one, please do all you can to keep him happy because a good finisher is out to please the hand that feeds him.

From contributor F:
Here's my schedule Lou:
Sand MDF with 100
1 coat Clawlock
Sand with 180
1 more coat of Clawlock
Sand with 220
1 coat Resistant
Sand with 320
Last coat Resistant

If I need a gloss finish, I use 500 Abralon after the 320 sanding.